1. Field of the Invention
The invention relates to a process for the preparation of coated granular sodium percarbonate having a low TAM value, and to a product obtainable by the process, which product is distinguished by a low TAM value (microcalorimetric determination of the release of energy during storage). The process according to the invention comprises (i) the preparation of granular sodium percarbonate by fluidized-bed spray granulation, and (ii) coating of the granular sodium percarbonate with a coating component in a fluidized bed.
2. Discussion of the Background
For the preparation of sodium percarbonate of the general formula 2 Na2CO3.3H2O2, which is used as a bleaching component in washing and cleaning agents, crystallization processes and fluidized-bed spray granulation processes in particular are used on an industrial scale.
Although sodium percarbonate obtained by crystallization processes can readily be stored owing to its often low TAM value per se, the active oxygen stability in the presence of washing agent constituents, such as, especially, zeolites, is unsatisfactory because of the porous surface. Although the stability of sodium percarbonate produced by a crystallization process to storage in the presence of washing agent constituents can be improved by coating the sodium percarbonate core with components having a stabilizing action, the demands nowadays made of such a product are often no longer adequately met.
Fluidized-bed spray granulation processes yield substantially spherical, dense sodium percarbonate particles which have a shell-like structure, resulting from the preparation, and higher storage stability than a product obtained by crystallization. In order to carry out the process, an aqueous hydrogen peroxide solution and an aqueous soda solution or, optionally, soda suspension are sprayed into a fluidized-bed containing sodium percarbonate particles whose diameter is smaller than that of the particles to be prepared. During the spraying in of the reactants, which are in an aqueous medium, water is evaporated off at a fluidized-bed temperature in the range from 40 to 95° C. Details regarding the implementation of the fluidized-bed spray granulation process are to be found, for example, in EP patent 0 716 640. In order further to increase the active oxygen stability in the presence of washing agent constituents, sodium percarbonate produced by fluidized-bed spray granulation can also be coated with a stabilizing coating layer, for example with a sodium sulfate layer according to EP patent 0 863 842.
For reasons of safety when handling sodium percarbonate, especially increased safety during storage in a silo, there is an increased demand for sodium percarbonate having further improved storage stability, corresponding to a further reduced TAM value as compared with hitherto. Although the TAM value of sodium percarbonate can be lowered to a certain extent by coating the sodium percarbonate with an inactivating material, the effect that can be achieved thereby is in many cases still inadequate. The TAM value is a microcalorimetric determination of the release of energy during storage, determined by means of a TAM® Thermal Activity Monitor from Termometric AB, Järfälla (SE).
Various attempts have already been made to lower the TAM value of sodium percarbonate in the case of preparation by fluidized-bed spray granulation. As the inventors of the present Application have found, a sodium percarbonate prepared according to DE-OS 27 33 935, which was produced by fluidized-bed spray granulation, has a relatively low TAM value if both a condensed phosphate and magnesium sulfate are added to the solutions for spraying. A disadvantage of that process is the large amount of those added substances that is required.
DE patent application 100 48 514.6, which has not yet been published, teaches a further process for the preparation of sodium percarbonate having a low TAM value, preferably less than 6 μW/g, by fluidized-bed spray granulation, in which process, for the purpose of lowering the TAM value, there is preferably added to at least one of the solutions for spraying a magnesium compound in an amount of approximately from 100 to 1000 ppm Mg2+ and/or a selected chelating agent, such as an aminophosphonic acid, in an amount of from 200 to 1000 ppm. As already mentioned at the beginning, a sodium percarbonate having a low TAM value must generally also be covered with a stabilizing coating in order to ensure adequate storage stability of the active oxgyen content in washing and cleaning agents. While the TAM value is a criterion for safety during storage in a silo, the active oxygen stability is a criterion that is of importance especially for the storage of a washing agent containing sodium percarbonate.
Accordingly, the object of the present invention is to provide a process for the preparation of coated granular sodium percarbonate having a low TAM value, especially a TAM value of less than 8 μW/g, preferably less than 5 μW/g. The process should be as simple as possible to implement industrially. Preferably, no process steps should be required other than those already used in the case of known fluidized-bed spray granulation and coating in a fluidized bed.